1. Field of the Invention
This invention relates to a scanning tunneling microscope and more specifically to an improved scanning tunneling microscope for investigating magnetic properties of the surface of a specimen.
2. Description of the Related Art
With the progress of thin film deposition technology, particularly with the remarkable progress in MBE (molecular beam epitaxial process) and CVD (chemical vapor deposition process), it has become possible to form a magnetic film several nanometers thick and these processes show great promises for use in various fields. The magnetic properties of a film with thickness of one to several atomic layers are not necessarily identical with those of bulk material. Therefore, it is necessary to measure the magnetic properties of the individual magnetic films. The same thing can be said of surface magnetism. In other words, the surface of a magnetic substance shows magnetic properties different from those of the bulk.
For measurement of surface magnetism, there is a requirement that the method used should be sensitive both to surface and the electron spins. In this application the surface magnetism is used in a wide sense which includes both surface properties of magnetic material and the magnetism of thin films. Among the conventional technologies that can meet the above-mentioned requirement are the polarized photoelectron spectroscopy and polarized LEED (low energy electron diffraction). These methods are well known and there have been some study results obtained by use of these methods. Nevertheless, there remain problems yet to be solved.
To mention some of the problems, both the polarized photoelectron spectroscopy and the polarized LEED have low spatial resolution. Information that can be obtained through measurement by the polarized photoelectron spectroscopy and the polarized LEED is confined to the average properties in the range of several tens of nanometer at most. However, in many of important magnetic substances, electron spins are localized. For this reason, it is very important to observe the distribution of electron spins on an atomic-scale spatial resolution.
All of the above-mentioned technologies require a large-scale setup and considerable cost. The polarized LEED requires a GaAs polarized electron source. This GaAs polarized electron source is of great size that utilizes optical pumping by the laser. The polarized photoelectron spectroscopy, on the other hand, uses a Mott detector. This Mott detector is a large-size detector having an electrostatic accelerator at the preceding stage.
Meanwhile, in contrast with these technologies, a scanning tunneling microscope (STM) which has recently been developed is an apparatus that permits observation of the surface of a specimen on the atomic scale. When a metal tip is brought as close as about 1 nm to the specimen surface and a bias voltage is applied across the specimen and the tip a tunnel current flows between the specimen surface and the tip The STM is an apparatus for measuring the corrugation of the surface of the specimen by utilizing a phenomenon that the tunnel current changes in response to even slight changes in the distance between the tip and the specimen. With the STM, by a voltage applied between the tip and the specimen, it is possible to observe how surface electrons possessing a specific level of energy are distributed in the space. To take GaAs for example, the electrons of GaAs are mostly localized in As atoms. The STM is capable of discriminatingly imaging the state where electrons are localized in As atoms and the state where there is no electron in Ga atoms. However, from measurement by the conventional STM, it is impossible to obtain information as to electron spins.
As is clear from the foregoing, measuring technologies which are sensitive to the electron spins are inferior in spatial resolution. And, the STM superior in spatial resolution is unable to measure the distribution of electron spins.